U.S. patent number 4,498,739 [Application Number 06/381,800] was granted by the patent office on 1985-02-12 for electrochromic display devices using iron(iii) hexacyanoferrate(ii) salt.
This patent grant is currently assigned to Seiko Instruments & Electronics Ltd.. Invention is credited to Tatsuaki Ataka, Kingo Itaya, Koji Iwasa, Kimio Shibayama, Shinobu Toshima.
United States Patent |
4,498,739 |
Itaya , et al. |
February 12, 1985 |
Electrochromic display devices using iron(III) hexacyanoferrate(II)
salt
Abstract
An electrochromic display device comprises an electrolyte
contained between a pair of spaced-apart substrates. A set of
electrodes are disposed on at least one of the substrates in
contact with the electrolyte and a layer of electrochromic material
is disposed on at least one of the electrodes to define a display
electrode. The electrochromic material comprises a color-reversible
salt of iron(III) hexacyanoferrate(II) which, depending on its
oxidation-reduction state, exhibits different colors. A source of
electric charge is connected through circuitry to effect reversible
electrochemical oxidation and reduction of the salt of iron(III)
hexacyanoferrate(II) to effect a corresponding reversible color
change exhibited by the display electrode.
Inventors: |
Itaya; Kingo (Tagajo,
JP), Shibayama; Kimio (Sendai, JP),
Toshima; Shinobu (Sendai, JP), Ataka; Tatsuaki
(Tokyo, JP), Iwasa; Koji (Tokyo, JP) |
Assignee: |
Seiko Instruments & Electronics
Ltd. (Tokyo, JP)
|
Family
ID: |
27303139 |
Appl.
No.: |
06/381,800 |
Filed: |
May 25, 1982 |
Foreign Application Priority Data
|
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|
|
|
May 26, 1981 [JP] |
|
|
56-79911 |
Oct 2, 1981 [JP] |
|
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56-156971 |
Dec 7, 1981 [JP] |
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56-196623 |
|
Current U.S.
Class: |
359/273; 204/292;
205/316; 359/266 |
Current CPC
Class: |
C09K
9/00 (20130101); G02F 1/1516 (20190101); G02F
2001/1517 (20130101) |
Current International
Class: |
G02F
1/15 (20060101); C09K 9/00 (20060101); G02F
1/01 (20060101); G02F 001/01 (); C25D 011/00 ();
C25B 011/04 () |
Field of
Search: |
;350/357 ;340/705
;204/112,56R,29R,292 |
Foreign Patent Documents
Other References
Vernon D. Neff, J. Electrochem. Soc.: Electrochemical Science &
Technology, 125, No. 6, 886-887 (1978)..
|
Primary Examiner: Arnold; Bruce Y.
Assistant Examiner: Propp; William
Attorney, Agent or Firm: Burns; Robert E. Lobato; Emmanuel
J. Adams; Bruce L.
Claims
What we claim is:
1. In an electrochromic display device: a pair of spaced-apart
substrates; an electrolyte contained between the spaced-apart
substrates; a set of electrodes disposed on at least one of the
substrates in contact with the electrolyte; a layer of
electrochromic material disposed on at least one of the electrodes
to define a display electrode, the layer of electrochromic material
comprising a color-reversible salt of iron(III)
hexacyanoferrate(II) electrodeposited on the surface of the display
electrode as an intimately adhering blue insoluble solid film; and
means for effecting reversible electrochemical oxidation and
reduction of the salt of iron(III) hexacyanoferrate(II) to effect a
corresponding reversible color change exhibited by the display
electrode.
2. The electrochromic display device as set forth in claim 1;
wherein the layer of electrochromic material comprises a layer of
iron(III) hexacyanoferrate(II) salt electrodeposited on the surface
of the display electrode as an intimately adhering blue insoluble
solid film from a solution having iron(III) ions and
hexacyanoferrate(III) ions dissolved therein.
3. The electrochromic display device as set forth in claim 1;
wherein said layer of electrochromic material exhibits a first
color state in the ground state, a second color state different
than the first color state in the reduced state and a third color
state different from the first and second color states in the
oxidized state.
4. The electrochromic display device as set forth in claim 1;
wherein the layer of the electrochromic material disposed on the
display electrode is a layer of a complex salt composition
containing iron(III) hexacyanoferrate(II) which
in a ground state exhibits a first color state and has a
composition corresponding to the formula ##EQU16## in a reduced
state exhibits a second color state and has a composition
corresponding to the formula ##EQU17## and in an oxidized state
exhibits a third color state and has a composition corresponding to
the formula ##EQU18## wherein M.sup.+ is a monovalent cation,
X.sup.l- is an l-valent anion,
l is 1 or 2,
m is 0 or a positive integer of not more than 5,
n is 0 or a positive integer of not more than 14,
x and y are positive integers,
and wherein l, m, x and y satisfy all the following
relationships,
5. The electrochromic display device as set forth in claim 1;
wherein said layer of the electrochromic material is an insoluble
coating layer which
in the ground state is insoluble Prussian blue represented by the
formula
in the reduced state is represented by the formula
and in the oxidized state is represented by the formula ##EQU19##
wherein M.sup.+ is a monovalent cation
X.sup.l- is an anion of valence l.sup.-
l is 1 or 2.
6. The electrochromic display device as set forth in claim 4 or 5;
wherein the monovalent cation M.sup.+ is a potassium ion, rubidium
ion, cesium ion or ammonium ion; and the electrolyte comprises an
electrolyte containing the same cation.
7. The electrochromic display device as set forth in claim 4 or 5;
wherein the anion X.sup.l- is a fluoride ion, chloride ion, nitrate
ion, perchlorate ion, tetrafluoroborate ion or hexafluorophosphate
ion when l is equal to 1, or a sulfate ion or carbonate ion when l
is equal to 2; and the electrolyte comprises an electrolyte
containing the same anion.
8. The electrochromic display device as set forth in any one of
claims 1-5 wherein the electrolyte comprises an electrolyte
solution having a pH value in the range of between about 3 and
5.
9. The electrochromic display device as set forth in claim 1;
wherein the set of electrodes other than the display electrodes
includes a counter electrode disposed on the substrate opposite to
the substrate having disposed thereon the display electrodes; and a
layer of a salt of iron(III) hexacyanoferrate(II) disposed on the
counter electrode.
10. The electrochromic display device as set forth in claim 9;
wherein the counter electrode comprises an electrode coated with a
layer of a salt of iron(III) hexacyanoferrate(II) which is
electrodeposited as an insoluble solid film from a solution
containing iron(III) ions and hexacyanoferrate(III) ions.
11. A method of producing a display element for use in an
electrochromic display device comprising: providing an aqueous
solution containing iron(III) ions and hexacyanoferrate(III) ions,;
and electrodepositing iron(III) hexacyanoferrate(II) salt on an
electrode from the aqueous solution containing iron(III) ions and
hexacyanoferrate(III) ions.
12. The method of claim 11; wherein the aqueous solution contains
about equal molar amounts of iron(III) ions and
hexacyanoferrate(III) ions.
13. The method of claim 11; wherein the aqueous solution comprises
an aqueous acidic solution.
14. The method of claim 11; wherein the aqueous solution contains
about equal molar amounts of iron(III) chloride and potassium
hexacyanoferrate(III).
15. In an electrochromic display device: a display element
comprising an electrode, and an electrochromic material comprised
of a layer of iron(III) hexacyanoferrate(II) salt electrodeposited
on the surface of said electrode as an intimately adhering blue
insoluble solid film.
16. The electrochromic display device according to claim 15;
wherein the layer of electrochromic material comprises a layer of
iron(III) hexacyanoferrate(II) salt electrodeposited on the surface
of the electrode as an intimately adhering blue insoluble solid
film from a solution having iron(III) ions and
hexacyanoferrate(III) ions dissolved therein.
17. The electrochromic display device according to claim 15;
wherein said layer of electrochromic material is an insoluble
coating layer which
in the ground state is insoluble Prussian blue represented by the
formula
in the reduced state is represented by the formula
and in the oxidized state is represented by the formula ##EQU20##
wherein M.sup.+ is a monovalent cation
X.sup.l.spsp.- is an anion of valence l.sup.-
l is 1 or 2.
18. A display electrode for use in an electrochromic display device
comprising: an electrode; and an electrochromic material comprised
of a layer of iron(III) hexacyanoferrate(II) salt electrodeposited
on the surface of said electrode as an intimately adhering blue
solid film in an amount effective to exhibit electrochromic
activity.
19. The display electrode according to claim 18, wherein the layer
of electrochromic material comprises a layer of iron(III)
hexacyanoferrate(II) salt electrodeposited on the surface of the
electrode as an intimately adhering blue insoluble solid film.
20. The display electrode according to claim 18; wherein the layer
of electrochromic material comprises a layer of iron(III)
hexacyanoferrate(II) salt electrodeposited on the surface of the
electrode as an intimately adhering blue insoluble solid film from
a solution having iron(III) ions and hexacyanoferrate(III) ions
dissolved therein.
21. The display electrode according to claim 18; wherein said
electrochromic material
in a ground state exhibits a first color state and has a
composition corresponding to the formula ##EQU21## in a reduced
state exhibits a second color state and has a composition
corresponding to the formula ##EQU22## and in an oxidized state
exhibits a third color state and has a composition corresponding to
the formula ##EQU23## wherein M.sup.+ is a monovalent cation,
X.sup.l- is an l-valent anion,
l is 1 or 2,
m is 0 or a positive integer of not more than 5,
n is 0 or a positive integer of not more than 14,
x and y are positive integers,
and wherein l, m, x and y satisfy all the following
relationships,
22. The display electrode according to claim 18; wherein said layer
of the electrochromic material is an insoluble coating layer
which
in the ground state is insoluble Prussian blue represented by the
formula
in the reduced state is represented by the formula
and in the oxidized state is represented by the formula ##EQU24##
wherein M.sup.+ is a monovalent cation
X.sup.l- is an anion of valence l.sup.-
l is 1 or 2.
23. The display electrode according to claim 21 or 22; wherein the
monovalent cation M.sup.+ is a potassium ion, rubidium ion, cesium
ion or ammonium ion; and the electrolyte comprises an electrolyte
containing the same cation.
24. The display electrode according to claim 21 or 22; wherein the
anion X.sup.l- is a fluoride ion, chloride ion, nitrate ion,
perchlorate ion, tetrafluoroborate ion or hexafluorophosphorate ion
when l is equal to 1, or a sulfate ion or carbonate ion when l is
equal to 2; and the electrolyte comprises an electrolyte containing
the same anion.
25. A process for synthesizing iron(III) hexacyanoferrate (II)
comprising the steps of immersing a pair of electrodes in a
solution mixture of an iron(III) ion-containing solution and a
hexacyanoferrate(III) ion-containing solution, and effecting
electrolysis with one of said pair of electrodes being an anode and
the other being a cathode, thereby depositing iron(III)
hexacyanoferrate(II) as a blue electrolytic product on the surface
of the cathode.
26. A film of iron(III) hexacyanoferrate(II) synthesized by process
of immersing a pair of electrodes in a solution mixture of an
iron(III) ion-containing solution and a hexacyanoferrate(III)
ion-containing solution, and effecting electrolysis with one of
said pair of electrodes being an anode and the other being a
cathode, thereby depositing iron(III) hexcyanoferrate (II) as a
blue electrolytic product on the surface of the cathode.
27. An electrodeposited film or iron(III) hexacyanoferrate (II) on
an electrode.
28. An electrodeposited film of iron(III) hexacyanoferrate (II)
according to claim 27, wherein said electrode is effective as the
cathode in the electrolysis of a solution containing iron(III) ions
and hexacyanoferrate(III) ions.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to electrochromic display devices
utilizing the reversible color change of an electrochromic material
due to electrochemical oxidation-reduction reaction, and more
particularly, to electrochromic display devices wherein the
electrochromic material comprises iron(III) hexacyanoferrate(II)
salt.
With the advances being made in electronics technology, there is an
increasing demand for display devices for use with portable
information processing instruments, such as watches and electronic
calculators. Conventional display devices used for these purposes
usually include liquid crystal display devices. Although liquid
crystal display devices have the advantages of fairly acceptable
response speed and a somewhat reliable lifetime, they have the
serious disadvantages of exhibiting dark display and reduced
visibility when viewed at an angle (visual dependence). Liquid
crystal devices also have visual and aesthetic limitations which
restrict their use in some types of display devices. To provide a
new display device which is free of the shortcomings of liquid
crystal display devices, considerable research and development have
been carried out involving electrochromic display devices utilizing
the phenomenon of reversible color change which occurs in
particular materials through electrochemical oxidation and
reduction. Typical examples of such electrochemically
color-developing materials (which are referred to hereinafter as
"electrochromic materials") used in the prior art are viologen,
which is composed of alkyl quaternary ammonium derivatives of
.gamma.,.gamma.'-dipyridyl, and transition metal oxides, as
exemplified by tungsten trioxide. The electrochromic material is
applied as a layer or film onto the surface of an electrode to form
what is commonly known in the art as a "display element" or
"display electrode" and for convenience of description, the term
"display electrode" is primarily used in the following description
of the invention.
The present invention is discriminated from the prior art
techniques in that it provides an electrochromic display device
using a salt of iron(III) hexacyanoferrate(II) which has not
heretofore been used as an electrochromic material.
In the case of viologen, which is a typical example of a
conventional electrochromic material, when an electrolyte solution
having viologen dissolved therein is subjected to electrolytic
reduction, a viologen radical is formed as a colored deposit on the
cathode surface according to the reaction of equation (1): ##STR1##
wherein R is an alkyl group. Such systems, which involve a reaction
resulting in the deposit of a coloring species from an electrolyte
solution onto the electrode surface, have the following
disadvantages: the quantity of charge must be precisely controlled
per unit area of the electrode to provide a uniform tone, because
the display color depends on the quantity of the deposit formed on
the electrode surface, and they tend to deteriorate after repeated
oxidation-reduction cycles, as the viologen itself lacks chemical
stability.
Another conventional system using tungsten oxide, one of the
transition metal oxides, as an electrochromic material, utilizes
color development by tungsten bronze. The tungsten bronze is
produced through a reaction caused by the simultaneous introduction
of electrons from the electrode, and metal ions from the
electrolyte solution, into the layer of tungsten oxide on the
electrode surface, according to equation (2): ##EQU1##
In order that the reaction of equation (2) be electrochemically
reversible, x should be equal to or less than 0.3 (x.ltoreq.0.3) in
equation (2). If x is more than 0.3 (x>0.3), the reaction
becomes irreversible and thus incompatible with the display
purpose. Display devices involving a non-stoichiometric reaction,
such as required to effect the reversible production of tungsten
bronze, are difficult to drive because the electrolysis must always
be controlled so that x does not exceed the reversible range, and
thus these prior art display device are not suitable for all
commercial applications.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a new
electrochromic display which overcomes the afore-mentioned problems
of prior art display devices.
A further object of the present invention is to provide a stable
electrochromic display device which utilizes a stoichiometric
electrochemical reaction.
Another object of the present invention is to provide a new display
element or display electrode composed of a new type of
electrochromic material.
Still another object of the present invention is to provide a new
process for producing display electrodes for use in electrochromic
display devices.
These and other objects of the invention are achieved by a display
device comprised of a display electrode having an iron(III)
hexacyanoferrate(II) salt as the electrochromic reactive
material.
A unique method of preparing a thin film of iron(III)
hexacyanoferrate(II) salt has been discovered whereby control of
the optical density is readily achieved. Moreover, a relatively
simple display device can be obtained by using iron(III)
hexacyanoferrate(II) salt as the electroactive material for both
the display and counter electrodes.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1-(a) is an explanatory diagram of the crystal lattice of
insoluble Prussian blue, and FIG. 1-(b) is an explanatory diagram
of the crystal lattice of soluble Prussian blue. In the figure, the
symbol designates Fe.sup.II, designates Fe.sup.III, and in FIG.
1-(b), the C and N atoms of some of the cyano groups are shown
bonding with Fe.sup.II and Fe.sup.III, respectively. The water
molecules in the crystal are omitted for clarity of illustration as
are all of the cyano groups in FIG. 1-(a) and some of the cyano
groups in FIG. 1-(b).
FIG. 2 is a cyclic voltammogram obtained when an SnO.sub.2
electrode coated with about 2.5 mC./cm..sup.2 of an iron(III)
hexacyanoferrate(II) salt according to the present invention is
driven in 1M KCl.
FIG. 3 shows the absorption spectra of an SnO.sub.2 transparent
electrode coated with about 10.5 mC./cm..sup.2 of an iron(III)
hexacyanoferrate(II) salt according to the present invention when
it is at 1.35 V, 0.65 V and -0.2 V vs. S.C.E., respectively.
FIG. 4 is a cyclic voltammogram of the iron(III) hexacyanoferrate
(II) salt-coated electrode of the present invention having a peak
at about 0.2 V, and the integral value of the charge quantity
required for oxidation-reduction along this wave is shown by dashed
lines. In the FIG., 1 corresponds to the first cycle and 10.sup.4
corresponds to the end of 10.sup.4 cycles.
FIG. 5 shows the reactive film retention of the iron(III)
hexacyanoferrate(II) salt-coated electrode of the present invention
at varying pH, in which a solid line designates the reactive film
retention after 10.sup.5 cycles and a dashed line designates the
reactive film retention of a similar electrode after immersion for
about 24 hours.
FIG. 6 shows the size of the bottle-neck of Prussian blue.
FIG. 7 shows the variation of the reactive film retention with
increasing cycles for various cations.
FIG. 8 is a cyclic voltammogram of the iron(III)
hexacyanoferrate(II) salt-coated electrode of the present
invention, in which (1) corresponds to the electrode in 1M KCl and
(2) and (3) correspond to the electrode in 1M NaCl.
FIG. 9 is a cyclic voltammogram of the iron(III)
hexacyanoferrate(II) salt-coated electrode of the present invention
in 0.1N KBF4, in which Q.sub.1 designates the quantity of charge
required for oxidation-reduction along the wave having a peak at
about 0.2 V vs. S.C.E. and Q.sub.2 designates the quantity of
charge required for oxidation-reduction along the wave having a
peak at about 0.9 V vs. S.C.E.
FIG. 10 shows the variation of the reactive film retention with
increasing cycles for various anions.
FIG. 11-(a) is a plan view and FIG. 11-(b) is a cross-sectional
view of one embodiment of display device constructed in a flat
form.
FIG. 12-(a) is a plan view and FIG. 12-(b) is a cross-sectional
view of an embodiment of electrochromic numerical display device
according to the present invention.
FIG. 13 is a sectional view of another embodiment of electrochromic
display device according to the present invention.
FIG. 14 is a sectional view of a further embodiment of
electrochromic display device according to the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention a salt of iron(III)
hexacyanoferrate(II) has been found to be an improved, novel
electrochromic material which is free of many of the disadvantages
of the conventional electrochromic materials. In general, the
iron(III) hexacyanoferrate(II) salt is included in a group of blue
pigment compounds known as Prussian blue. At present, the group of
compounds called Prussian blue are generally classified into two
groups of compounds: insoluble Prussian blue and soluble Prussian
blue of formulas (4) and (5), respectively.
wherein M.sup.+ is a monovalent cation such as Li.sup.+, Na.sup.+,
K.sup.+, Rb.sup.+, Cs.sup.+, NH.sub.4.sup.+, etc.
FIG. 1-(a) and FIG. 1-(b), show the crystalline structures of
insoluble Prussian blue and soluble Prussian blue, respectively. As
is apparent from FIG. 1, both insoluble Prussian blue and soluble
Prussian blue are mixed valence complexes having a
three-dimensional network structure in which a cyano group links
Fe.sup.II and Fe.sup.III. The C atom of the cyano group coordinates
with Fe.sup.II and the N atom of the cyano group coordinates with
Fe.sup.III. Iron atoms of different oxidation numbers, that is,
Fe.sup.III and Fe.sup.II coexist in a common compound, and for this
reason, the compound is called a mixed valence complex. The
aesthetic blue color inherent to Prussian blue is attributable to
the mixed valence absorption band due to the coexistence of iron
atoms of different oxidation numbers in the single compound as
described above. If all the Fe.sup.III atoms in the crystal are
reduced to Fe.sup.II, the crystal loses its blue color and becomes
colorless, while the crystalline structure itself remains
unchanged.
The present invention is based on the discovery that the valence of
Fe.sup.III in the above-mentioned mixed valence complex may be
reversibly changed between trivalence and divalence by an
electrochemical oxidation-reduction with a concomitant change in
color in an electrochromic display device. This oxidation reduction
reaction may be represented by equation (6) or (6'): ##EQU2##
wherein M.sup.+ represents a monovalent cation such as Li.sup.+,
Na.sup.+, K.sup.+, Rb.sup.+, Cs.sup.+, NH.sub.4.sup.' , etc.
Unlike the earlier-mentioned viologen system which depends on the
cyclic deposition and dissolution of a coloring species from and
into solution, an electrochromic display element having an
iron(III) hexacyanoferrate(II) salt incorporated as an
electrochromic material utilizes the color change which occurs as a
result of an electrochemical reaction of the iron(III)
hexacyanoferrate(II) salt, which is present on the surface of the
display electrode as a contiguous, uniform thin film of an
insoluble mixed valence complex having a three-dimensional network
structure. Consequently, the color is always developed at a given
color density depending on the thickness of the iron(III)
hexacyanoferrate(II) salt layer. Furthermore, as shown by equations
(6) and (6'), the reaction is not a non-stoichiometric reaction as
in the case of reaction of tungsten oxide according to equation
(2), but rather a stoichiometric one or four-electron transfer
reaction. The display element of the invention is easy to drive
because the quantity of the charge does not need to be precisely
controlled.
There is no chemical stability problem with respect to the blue
pigment known as Prussian blue as shown by its long, established
use in a wide variety of applications, for example, in paint and
printing ink, since its discovery in 1704. The electrochromic
display element of the invention, comprised of iron(III).
hexacyanoferrate(II) salt is easy to drive to develop an aesthetic
Prussian blue color at a given density and involves a particular
coloring species having well established chemical stability proved
by long usage.
The iron(III) hexacyanoferrate(II) salt used as the electrochromic
material in the present invention preferably corresponds to the
compound traditionally called insoluble Prussian blue and having
the chemical formula: Fe.sub.4 [Fe(CN).sub.6 ].sub.3.nH.sub.2 O
wherein n is equal to 12 to 14. As mentioned above, the compound
called Prussian blue is generally classified into two
compounds--namely, insoluble Prussian blue and soluble Prussian
blue. Each of these compounds is derived by particular respective
processes.
Insoluble Prussian blue may be synthesized by mixing an aqueous
solution containing hexacyanoferrate(II) ions with an excess of a
solution containing iron(III) ions and causing insoluble Prussian
blue to precipitate from the resulting mixture.
On the other hand, soluble Prussian blue may be synthesized by
mixing equal molar amounts of an aqueous solution of
hexacyanoferrate(II) ions and a solution of iron(III) ions and
causing soluble Prussian blue to precipitate from the resulting
mixture.
Unlike the two known processes, a unique process has been
discovered for synthesizing the display element of the invention in
which iron(III) hexacyanoferrate(II) salt is electrochemically
synthesized or electrodeposited directly on the surface of a
display electrode as an intimately adhering, contiguous, uniform
film. This inventive process provides a display electrode capable
of electrochemical reversible color change.
Preferably, the electrochromic display device of the invention is
prepared by electrodepositing iron(III) hexacyanoferrate(II) salt
on a substrate as a thin uniform film from a solution containing
iron(III)ions and hexacyanoferrate(III) ions, for example, from a
solution of potassium hexacyanoferrate(III) and ferric chloride. An
aqueous acid solution containing approximately equal amounts of
hexacyanoferrate(III) ion and iron(III) ion is partilarly
preferred. The resulting deposit is a solid, adhering film of the
formula, Fe.sup.III.sub.4 [Fe.sup.II (CN).sub.6 ].sub.3, of
insoluble Prussian blue. As a result of determinations described
below, the composition resulting from electrodeposition according
to the invention has been found to have a composition in the ground
state which more precisely corresponds to the formula: ##EQU3## in
the reduced state the composition corresponds to the formula:
##EQU4## and in the oxidized state to the formula: ##EQU5##
wherein: M.sup.+ is a monovalent cation,
X.sup.l- is an l-valent anion,
l is 1 or 2,
m is 0 or a positive integer of not more than 5,
n is 0 or a positive integer of not more than 14,
x and y are positive integers,
and wherein l, m, x, and y satisfy the following relationships:
The electrochromic material in the display device of the invention
is preferably insoluble Prussian blue.
As demonstrated by determinations which are discussed below the
oxidation-reduction mechanism of insoluble Pressian blue is
different from that of soluble Prussian blue.
FIG. 2 is a cyclic voltammogram of an SnO.sub.2 transparent
electrode coated with about 2.5 mC./cm..sup.2 of the iron(III)
hexacyanoferrate(II) salt according to the present invention in 1M
KCl aqueous solution. FIG. 3 shows the absorption spectra of a
similar electrode which is at potentials of +1.35 V, +0.65 V and
-0.2 V vs. S.C.E. and at a charge density of 10.5 mC./cm..sup.2. It
has been found that this electrode exhibits a spectrum having an
absorption peak at 690 nm and is thus colored blue when it is at a
potential of +0.6 V vs. S.C.E. This spectrum having an absorption
peak at 690 nm substantially agrees with that previously reported
for colloidal Prussian blue. The electrode shows no absorption and
is transparent when it is at about -0.2 V vs. S.C.E., and it shows
a moderate absorption peak at 425 nm and is thus colored pale brown
when it is at about 1.35 V vs. S.C.E.
As seen from the cyclic voltammogram of FIG. 2, waves appearing in
this electrode have two peaks at about +0.2 V and +0.9 V vs. S.C.E.
For each of the peaks, the electrochemical reactions involved in
soluble and insoluble Prussian blue are considered below.
First, the electrochemical reactions of soluble and insoluble
Prussian blues are compared with respect to the wave having a peak
at about +0.2 V vs. S.C.E.
Reaction of soluble Prussian blue: ##EQU6##
Reaction of insoluble Prussian blue: ##EQU7## In the above
equations, M.sup.+ represents a monovalent cation such as Li.sup.+,
Na.sup.+, K.sup.+, R.sup.+, Cs.sup.+, NH.sub.4.sup.+, etc.
As used throughout the specification and drawings, the abbreviation
S.C.E. stands for Saturated Calomel Electrode which is widely used
as a reference electrode and which has a stable electric potential
of +0.246 V at 25.degree. C. with respect to the international
standard hydrogen electrode.
As seen from the above equations, the soluble and insoluble
Prussian blues give rise to electrochemical reactions which are
common in that an electron(s) and a monovalent cation(s) are
simultaneously introduced into the coating.
Next, the electrochemical reactions of soluble and insoluble
Prussian blues are compared with respect to the wave having a peak
at +0.9 V vs. S.C.E.
Reaction of soluble Prussian blue: ##EQU8##
Reaction of insoluble Prussian blue: ##EQU9## In the above
formulas, X.sup.l- represents an l-valent anion.
A comparison between equations (7) and (7') reveals that the
electrochemical reaction of soluble Prussian blue allows M.sup.+
cations and electrons to move in and out of the coating, whereas
the electrochemical reaction of the insoluble Prussian blue allows
X.sup.l- anions and electrons to move in and out of the coating. In
summary, the soluble and insoluble Prussian blues have two
significantly different features with respect to the wave having a
peak at about +0.2 V and the wave having a peak at about +0.9 V vs.
S.C.E. First, the wave having a peak at about +0.9 V vs. S.C.E.
induces an electrochemical reaction accompanying the movement of
M.sup.+ cations in the case of soluble Prussian blue, whereas the
same wave induces another electrochemical reaction accompanying the
movement of X.sup.l- anions in the case of insoluble Prussian blue.
The second difference is the ratio of electrons associated with the
waves having peaks at about +0.2 V and +0.9 V vs. S.C.E. The
electron ratio is 1:1 for soluble Prussian blue while it is 4:3 for
insoluble Prussian blue. As evident from the foregoing, it is
necessary to determine whether the iron(III) hexacyanoferrate(II)
used as the electrochromic material is soluble or insoluble
Prussian blue to properly drive an electrochromic display element
using iron(III) hexacyanoferrate(II) and therefore the associated
electrochemical reaction must be fully understood.
As illustrated by the following examples, an aspect of the present
invention is the establishment of proper principles for operating
an iron(III) hexacyanoferrate(II) salt-coated electrode; to this
end the composition of the iron(III) hexacyanoferrate(II) salt
obtained by electrodeposition has been determined through atomic
absorption and flame spectrochemical analyses, and a new
electrochemical oxidation-reduction mechanism has been found
through electrochemical measurements. While the present examples
are directed specifically to principles for operating a display
device having as the electrochromic material, an insoluble
iron(III) hexacyanoferrate(II) salt, it is apparent that the same
determinations and corresponding principles can be applied to the
operation of a display device having soluble iron(III)
hexacyanoferrate(II) salt as the electrochromic material.
EXAMPLE 1
This example illustrates a general process of preparing an
iron(III) hexacyanoferrate(II) salt-coated electrode to be used in
the present invention.
An aqueous solution containing 20 millimoles/liter of potassium
hexacyanoferrate(III) (K.sub.3 Fe(CN).sub.6) and an aqueous
solution containing 20 millimoles/liter of iron(III) chloride
(FeCl.sub.3) and adjusted to 0.2 N with hydrochloric acid were
mixed in equal volumes to form a clear brown solution.
Immersed in this clear brown solution were a platinum plate
electrode having an area of 1 cm..sup.2 which was to become an
iron(III) hexacyanoferrate(II) salt-coated display element or
display electrode of the present invention and a platinum plate
electrode having an area of about 10 cm..sup.2 for supplying
current to the display electrode. Galvanostatic electrolysis was
then effected for about 10 minutes by supplying a current of 10
.mu.A to cathodically polarize the 1 cm..sup.2 platinum plate
electrode. After the electrolysis was complete, the
cathodically-polarized platinum plate electrode of 1 cm..sup.2 in
area was removed from the clear brown solution it was found that a
contiguous, uniform, blue, insoluble solid film had been deposited
on the surface of the 1 cm..sup.2 platinum plate electrode.
EXAMPLE 2
In this example, the composition of the iron(III)
hexacyanoferrate(II) salt was investigated.
The general procedure of Example 1 used to coat an electrode with
the iron(III) hexacyanoferrate(II) salt was repeated, except that
the electrode to be coated was a platinum plate having an area of
10 cm..sup.2 and galvanostatic electrolysis was effected for 10
minutes with a current of 100 .mu.A. A blue insoluble solid film
was formed on the platinum cathode electrode of 10 cm..sup.2 in
area and used as a sample for analysis. The insoluble solid film
was dissolved in 28% aqueous ammonia, neutralized with 1/l
hydrochloric acid/water, and diluted to 100 cc. with water to
prepare a sample solution for analysis. Flame spectrochemical
analysis was used to determine the amount of potassium and atomic
absorption analysis was used to determine the amount of iron.
Another electrode coated with the iron(III) hexacyanoferrate(II)
salt was prepared separately from the electrode used above for
analysis. With this electrode placed in an aqueous solution of 1 M
KCl at pH 4.0, a cyclic voltammogram was measured to determine the
quantity of charge used in the oxidation-reduction reaction along
the wave having a peak at about +0.2 V vs. S.C.E.
Table 1 shows the quantities of potassium and iron determined by
flame spectrochemical and atomic absorption analyses in combination
with the corresponding quantity of electric charge.
TABLE 1 ______________________________________ unit: ug./mC.
Calculated for Calculated for Found Fe.sub.4 [Fe(CN).sub.6 ].sub.3
KFe[Fe(CN).sub.6 ] ______________________________________ Fe 1.02
1.01 1.16 K 0.06 0 0.41 ______________________________________
As seen from Table 1, the iron(III) hexacyanoferrate(II) salt
synthesized by the particular electrolytic process used in the
present invention is the so-called "insoluble Prussian blue",
corresponding to the composition of the formula: Fe.sub.4
[Fe(CN).sub.6 ].sub.3.
EXAMPLE 3
This example was carried out to determine the optimum pH range for
an electrolyte to be combined with the iron(III)
hexacyanoferrate(II) salt-coated electrode in an electrochromic
element according to the present invention.
FIG. 4 is a cyclic voltammogram of the iron(III),
hexacyanoferrate(II) salt-coated electrode of the present invention
in 1 M KCl aqueous solution, showing waves having a peak of about
0.2 V vs. S.C.E. The quantity of charge required for the
oxidation-reduction reaction at about 0.2 V vs. S.C.E. was measured
by means of a coulometer and is shown by dashed lines in FIG. 4.
These dashed lines show how the quantity of charge varies in
successive cycles when the iron(III) hexacyanoferrate(II)
salt-coated electrode is driven by applying a voltage of 1 Hz at
stepped potentials between 0.6 V and -0.2 V vs. S.C.E. The
variation of quantity of charge may be used as a measure of
evaluating the stability of the coating film, and the retention of
the reactive film is defined as follows: ##EQU10##
This reactive film retention was used to determine an adequate pH
range of the electrolyte required when the iron(III)
hexacyanoferrate(II) salt-coated electrode of the present invention
was operated. In the determination, aqueous solutions of 1 M KCl
were prepared and adjusted to varying pH's with hydrochloric acid,
and the retention of the reactive film was determined after
10.sup.5 cycles for each solution. The results are shown in FIG. 5,
in which a solid line shows how the reactive film retention varies
with pH when the electrodes are driven and a dashed line shows how
the reactive film retention varies with pH when the electrodes are
immersed. As seen from FIG. 5, the iron(III) hexacyanoferrate(II)
coated electrode of the present invention should be driven at
optimum pH in the range between 3 and 5. The iron(III)
hexacyanoferrate(II) coated electrode cannot be stably driven in an
alkaline electrolyte having a pH higher than 5 due to dissolution
of the coating film or an acidic electrolyte at a pH lower than 3
due to invasion of protons into the coating film.
EXAMPLE 4
This example was carried out to select the proper electrolyte
cation to be combined with the iron(III) hexacyanoferrate(II) salt
coated electrode in an electrochromic element according to the
present invention.
As previously mentioned, the wave having a peak at about 0.2 V vs.
S.C.E. is related to the movement of cations into and out of the
coating film. As shown in FIG. 1, the iron(III)
hexacyanoferrate(II) salt is of a substantially open structure
having a lattice constant of 10.2 .ANG. and any cations seem to
move freely into and out of such an open structure. However, when
the dimensions of the atoms constituting the crystal are taken into
account, the bottle-neck diameter, i.e. the maximum diameter of a
particle capable of moving freely through the crystal lattice, is
about 3.5 .ANG. as shown in FIG. 6. Thus the bottle-neck diameter
is a limit to the size of a cation which is capable of moving into
and out of the coating film.
In this example, electrolytes having different cations were used to
determine acceptable cations. The iron(III) hexacyanoferrate(II)
salt-coated electrode was prepared as in Example 1 and evaluated in
terms of the retention of the reactive film as in Example 3. The
electrolytes used were alkali metal chlorides, specifically LiCl,
NaCl, KCl, RbCl, and CsCl; an alkaline earth metal chloride,
specifically, BaCl.sub.2 ; and an ammonium salt, specifically
NH.sub.4 Cl. These aqueous electrolytic solutions were adjusted to
pH 4.0 and to a concentration of 0.1 N with hydrochloric acid, in
accordance with the results of Example 3. It has been found that
K.sup.+, Rb.sup.+, Cs.sup.+ and NH.sub.4.sup.+ ions provide for
stable coloring/bleaching of the coating film as shown by the data
in FIG. 7.
In conjunction with the above results, the radius of a crystallized
ion and the Stokes' ionic radius (which is one of measures of the
radius of a hydrated ion) are shown for various ions in Table 2. As
seen from this data, the Stokes' ionic radius of those cations
capable of stable driving is in good agreement with the bottle-neck
radius of 1.75 .ANG. of the iron(III) hexacyanoferrate(II)
salt.
TABLE 2 ______________________________________ Various ions and
their ionic radii Radius of Stokes' ionic Ion crystallized ion
(.ANG.) radius (.ANG.) ______________________________________
H.sup.+ 1.14 -- Li.sup.+ 0.60 2.37 Na.sup.+ 0.95 1.87 K.sup.+ 1.33
1.25 Rb.sup.+ 1.48 1.18 Cs.sup.+ 1.69 1.19 NH.sub.4.sup.+ 1.48 1.25
Ba.sup.++ 1.35 2.88 ______________________________________
EXAMPLE 5
This example was carried out to prove that the wave having a peak
at about 0.9 V vs. S.C.E. in the cyclic voltammogram does not
correspond to the electrochemical reaction based on the movement of
cations into and out of the iron(III) hexacyanoferrate (II) salt
coating film. A comparison of the quantity of charge in the
electrode reaction associated with the wave having a peak at about
0.2 V vs. S.C.E. with the quantity of charge in the electrode
reaction at about 0.9 V vs. S.C.E. will prove that the
electrochemical reaction at 0.9 V vs. S.C.E. is based on the
movement of anions into and out of the insoluble Prussian blue
layer.
FIG. 8 shows a cyclic voltammogram of an iron(III)
hexacyanoferrate(II) salt-coated electrode. Curve (1) in FIG. 8 is
a stable cyclic voltammogram of the electrode at about 0.2 V vs.
S.C.E. in 1 M KCl aqueous solution, and this is an electrode
reaction based on the movement of K.sup.+ ions as described in
Example 4. Curves (2) and (3) in FIG. 8 show cyclic voltammograms
of the electrode in 1 M NaCl. Curve (2) shows that the electrode
reaction at about 0.9 V vs. S.C.E. is stable and reversible, and
curve (3) shows that the electrode reaction at about 0.2 V vs.
S.C.E. gives rise to irreversible failure of the coating film after
one cycle of potential scanning. The reaction of curve (3) is based
on the movement of cations into and out of the electrode coating
film as described above and in this case, it results in failure of
the electrode coating film because the Stokes' radius of an
Na.sup.+ ion is larger than the bottle-neck diameter of a Prussian
blue crystal lattice. The reaction of curve (2) is reversible
unlike the reaction of curve (3), which indicates that this
electrode reaction is not based on the movement of Na.sup.+ ion
into and out of the electrode coating. Accordingly, the
electrochemical reaction represented by the cyclic voltammogram
having a peak at about 0.9 V vs. S.C.E. is not based on the
movement of cations into and out of the Prussian blue coating, but
probably on the movement of anions.
Further, in FIG. 9, a solid curve shows a cyclic voltammogram of
the iron(III) hexacyanoferrate(II) salt coating on a tin oxide
transparent electrode which is placed in an aqueous electrolyte
solution of 0.1N KBF.sub.4, and the dashed lines show how the
quantity of charge representative of the extent of electrode
reaction varies. The quantity of charge is measured as an integral
of the electrode current. In FIG. 9, Q.sub.1 is the quantity of
charge representative of the extent of an electrode reaction having
a peak at about 0.2 V vs. S.C.E., that is, the reacting weight
according to equation (6) or (6'). Q.sub.2 is the quantity of
charge representative of the extent of an electrode reaction having
a peak at about 0.9 V vs. S.C.E., that is, the reacting weight
according to equation (7) or (7'), provided that this electrode
reaction is based on the movement of anions into and out of the
electrode coating as concluded from the data shown in FIG. 8.
TABLE 3 ______________________________________ Comparison between
Q.sub.1 and Q.sub.2 Calculated for Calculated for Found Fe.sub.4
[Fe(CN).sub.6 ] KFe[Fe(CN).sub.6 ]
______________________________________ Q.sub.2 /Q.sub.1 .744 3/4 =
0.75 1/1 = 1.0 ______________________________________
The ratio of Q.sub.2 to Q.sub.1 measured in FIG. 9, the ratio of
reacting weights calculated from reaction formulas (6') and (7')
when the electrode coating film is insoluble Prussian blue
[Fe.sup.III.sub.4 [Fe.sup.II (CN).sub.6 ].sub.3, and the ratio of
reacting weights calculated from reaction formulas (6) and (7) when
the electrode coating film is soluble Prussian blue (KFe.sup.III
[Fe.sup.II (CN).sub.6 ]) are summarized in Table 3. As seen from
this data, there is support for the conclusion that the electrode
reaction exhibiting a peak at about 0.9 V vs. S.C.E. on the cyclic
voltammogram is a reaction associated with anions if the electrode
coating film is a film of insoluble Prussian blue.
EXAMPLE 6
This example was carried out to select an optimum anion for an
electrolyte when the results of Example 5 are taken into
account.
An iron(III) hexacyanoferrate(II) salt-coated electrode was
prepared as described in Example 1, and the evaluation of anions
was made on the basis of retention of the reactive film as in
Example 3. However, the reactive film retention based on the
quantity of electricity does not ensure a precise comparison
because the residual current has a considerable influence at
potentials in excess of 1.0 V vs. S.C.E. in the case of a
particular electrolyte. Thus, retention of the reactive film as
defined below was used as an index for evaluating the electrolyte
anions. That is, the ratio of peak currents at about 0.9 V vs.
S.C.E. was used in this example. ##EQU11##
The potential stepping waveform used for evaluation was a driving
waveform between 0.6 V and 1.4 V vs. S.C.E. having a frequency of 1
Hz. The electrolytes used were KCl, KF, KPF.sub.6, KNO.sub.3,
KClO.sub.4, CH.sub.3 COOK, K.sub.2 SO.sub.4 and KBF.sub.4.
FIG. 10 shows the retentions of reactive films obtained for various
anions on the wave having a peak at 0.9 V vs. S.C.E. in the cyclic
voltammograms of the electrodes through 10.sup.3 cycles.
Table 4 shows the Stokes' ionic radii of various ions. In Table 4,
the symbol `O` denotes acceptable film stability and the symbol `X`
denotes unacceptable film stability.
TABLE 4 ______________________________________ Various anions and
their Stokes' ionic radii Stokes' ionic Ion radius (A) Film
Stability ______________________________________ F.sup.- 1.67 0
Cl.sup.- 1.20 0 NO.sub.3.sup.- 1.29 0 ClO.sub.4.sup.- 1.36 0
BF.sub.4.sup.- -- 0 PF.sub.6.sup.- -- X CH.sub.3 COO.sup.- 2.24 X
SO.sub.4.sup.= 1.15 0 ______________________________________
As seen from FIG. 10 and Table 4, the anions which allow the
iron(III) hexacyanoferrate(II) salt-coated electrode of the present
invention to be operated in a stable manner are properly selected
on the basis of Stokes' radius of the anion and the bottleneck
diameter of the crystal lattice of the iron(III)
hexacyanoferrate(II) salt, as in the case of cations.
EXAMPLE 7
This example aimed at providing further information regarding the
precise principle of coloring/bleaching the iron(III)
hexacyanoferrate(II) salt-coated electrode according to the present
invention on the basis of a new discovery obtained by repeating
coloring/bleaching of the electrode between blue and colorless
states and thereafter analyzing the composition of the electrode
coating film.
The general procedure of Example 2 was repeated. The composition of
the coating film on the electrode was analyzed both immediately
after the preparation and after coloring/bleaching of the coated
electrode was carried out, for several cycles in an aqueous
solution of 1 M KCl.
TABLE 5 ______________________________________ Composition
distribution unit: .mu.g/mC As prepared After several cycles
______________________________________ Fe 1.02 1.02 K 0.06 0.30
______________________________________
As seen from Table 5, a comparison between the analytical values of
blue colored coating films after several cycles of
coloring/bleaching and the initially prepared film reveals that the
potassium content of the coating film increases after several
cycles. This means that potassium ions and electrons are
simultaneously introduced into the coating film during bleaching,
and further driving of the coating film in a coloring direction is
accompanied with not only the movement of potassium ions and
electrons out of the coating film, but also the movement of
chloride ions (anions) into and electrons out of the coating film
as shown by the following equations: ##EQU12##
This will be more generally expressed by electrochemical equation
(9): ##EQU13##
In the formulas of equation (9), M.sup.+, X.sup.l-, m, and x are
defined as follows.
M.sup.+ : a monovalent cation, K.sup.+, Rb.sup.+, Cs.sup.+,
NH.sub.4.sup.+, etc.
X.sup.l- : an l-valent anion, BF.sub.4.sup.-, SO.sub.4.sup.=,
ClO.sub.4.sup.-, etc.
m: 0, 1, 2, 3, 4 or 5
l: 1 or 2
x: the number of cations moving into and out of the coating
film.
Furthermore, the presence of either sub-lattices in a Prussian blue
lattice and other factors impose the following restriction to m, l,
and x. That is, m, l, and x should simultaneously satisfy the
relationships: ##EQU14##
EXAMPLE 8
This example provides a novel principle of coloring/bleaching the
iron (III) hexacyanoferrate(II) salt-coated electrode between blue
and brown colored states as a result of determinations described in
Example 7, on the wave having a peak at about 0.9 V vs. S.C.E. in
the cyclic voltammogram of the electrode.
The oxidation-reduction mechanism of the wave having a peak at
about 0.9 V may be represented by general equation (10):
##EQU15##
In the formulas of equation (10), M.sup.+, X.sup.l-, m, and y have
the following meanings.
M.sup.+ : a monovalent cation, such as K.sup.+, Rb.sup.+, Cs.sup.+,
NH.sub.4.sup.+, etc.
X.sup.l- : an l-valent anion, such as BF.sub.4.sup.-,
SO.sub.4.sup.=, ClO.sub.4.sup.-, etc.
m: 0, 1, 2, 3, 4 or 5.
l: 1 or 2.
y: the number of anions moving into and out of the coating
film.
EXAMPLE 9
A display device as shown in FIG. 11 was used in this example. FIG.
11-(a) and FIG. 11-(b) are plan and cross-sectional views of the
display device, respectively. A patterned tin oxide transparent
conductive film electrode 18 was formed on a glass substrate 17,
and a layer 19 of iron(III) hexacyanoferrate(II) was
electro-deposited on a display portion of the transparent
conductive film by a method similar to that described in Example
(1) to form a display electrode.
During this electrodeposition, the remaining portion of the
transparent conductive film 18 other than the display portion was
previously masked through the application of an adhesive cellophane
tape to prevent the electrochromic material from depositing
thereon. A conductive film electrode 21 of platinum was sputtered
on the surface of a lower glass substrate 20. The sputtered
platinum film 21 was coated on the surface with a layer 22 of
iron(III) hexacyanoferrate(II) salt in the same manner as in the
case of the display portion to form a counter electrode. An
electrolyte solution 23 comprising a solution of 1 molar potassium
chloride in 0.001N hydrochloric acid was placed between the coated
platinum film 21 and the coated glass substrate 17. A porous Teflon
sheet 24 was immersed in the electrolyte solution as a background
plate to constitute a white background for the display. The cell
was sealed along its periphery with a layer 25 of epoxy resin. A
voltage source 16 was connected to the two electrodes through a
double-throw switch 15 to operate the display device at a voltage
of 0.8 V.
When the contacts of the switch 15 were positioned at connection A,
a negative voltage of -0.8 V was applied to the display electrode
with respect to the counter electrode, and the iron(III)
hexacyanoferrate(II) salt layer 19 was changed into a colorless
state so that the display disappeared. When the switch 15 was
changed-over to connection C, the display electrode was at a
potential of 0 V with respect to the counter electrode, and the
iron(III) hexacyanoferrate(II) salt layer 19 turned blue to
establish the display. Furthermore, when the switch 15 was
changed-over to connection B, the electric circuit was opened and
no voltage was applied to the display electrode. The color of the
iron(III) hexacyanoferrate(II) salt layer 19 remained unchanged so
that either a colored state or a bleached state was maintained
depending on the state of the layer immediately before switching.
Consequently, the display state was memorized.
EXAMPLE 10
A display device as shown in FIG. 12 was used in this example.
FIGS. 12-(a) and 12-(b) are plan and cross-sectional views of the
display device which is shown as constituting an eight-segment
numerical display device. A tin oxide transparent conductive film
electrode 18 on an upper glass substrate 17 was provided with a
pattern of predetermined configuration by photolithography to form
a pattern of display electrodes. A lead portion of the transparent
conductive film other than the display portion was masked by vapor
depositing an insulating layer 26 of silicon dioxide before a layer
19 of potassium iron(III) hexacyanoferrate(II) was electrodeposited
on the display portion. On a lower glass substrate 20, an electrode
was formed as the counter electrode by coating a sputtered platinum
layer 21 with a layer 22 of potassium iron(III) hexacyanoferrate
partially containing reduced K.sub.2 Fe.sup.II [Fe.sup.II
(CN).sub.6 ]. A cell was filled with a solution of 1 molar
potassium chloride in 0.1N hydrochloric acid as an electrolytic
solution 23, equipped with white background plate 24 of porous
Teflon, and sealed along its periphery with a layer 25 of epoxy
resin. For electrical connection to the sputtered platinum layer
21, i.e., the counter electrode, a conductive layer 27 of
silver-dispersed conductive resin was formed in the epoxy resin
layer 25 at a corner of the cell to electrically connect the tin
oxide conductive film on the upper glass substrate to the patterned
lead portion of the counter electrode.
Though not shown in FIG. 12, the display device was connected to a
drive unit including a power supply, switching means for
electronically switching polarity, and a logic circuit coupled with
the switching means for selecting the polarity of the eight display
electrodes from among the positive, negative and open-circuit
states in accordance with a numeral to be displayed. The device was
operated by applying a voltage of 1 V. It was found that any
desired numeral could be displayed in an asethetic blue color by
controlling the switching of the electrical connections.
EXAMPLE 11
It is to be noted that the electrode arrangement is not limited to
the embodiments shown in FIG. 11 and FIG. 12. Other embodiments are
illustrated in FIG. 13 and FIG. 14. In these other embodiments, the
counter electrode has been removed. In the embodiment shown in FIG.
13, a plurality of display electrodes 19 are formed on a first
substrate 17. The electric charge which causes the electrochemical
oxidation and reduction reaction of the iron(III)
hexacyanoferrate(II) transfers between the display electrodes (as
shown by the double-headed arrow) so as to provide for display.
In the embodiment shown in FIG. 14, a plurality of display
substrate 19 are formed on both the first electrode 17 and the
second substrate 20. Then the charge which causes color change of
the iron(III) hexacyanoferrate(II) transfers between the display
electrodes on the first substrate and the display electrodes on the
second electrode (as shown by the double-headed arrow) so as to
provide for display.
* * * * *